The present application relates to a secondary battery provided with an electrolyte layer that includes electrolytic solution and a polymer compound. The present application also relates to a battery pack, an electric vehicle, an electric power storage system, an electric power tool, and an electronic apparatus that use the secondary battery.
In recent years, various electronic apparatuses such as a mobile phone and a mobile information terminal device (PDA) have been widely used, and it has been demanded to further reduce the size and the weight of the electronic apparatuses and to achieve their longer life. Accordingly, as an electric power source for the electronic apparatuses, a battery, in particular, a small and light-weight secondary battery capable of providing high energy density has been developed.
In these days, it has been considered to apply a secondary battery to various other applications in addition to the foregoing electronic apparatuses. Examples of such other applications may include a battery pack attachably and detachably mounted on the electronic apparatuses or the like, an electric vehicle such as an electric automobile, an electric power storage system such as a home electric power server, and an electric power tool such as an electric drill.
The secondary battery includes a cathode, an anode, and electrolytic solution. The electrolytic solution is generally mounted, on the secondary battery, being used to impregnate a separator therewith. Other than this, the electrolytic solution may be mounted, on the secondary battery, being held by a polymer compound, in some cases. The secondary battery in this case includes an electrolyte layer that is a so-called gel electrolyte. In the secondary battery provided with such an electrolyte layer, leakage of the electrolytic solution is prevented.
A configuration of the polymer compound included in the electrolyte layer may largely influence battery characteristics of the secondary battery, which has led to various considerations on the configuration of the polymer compound.
Specifically, in order to improve mobility of lithium ions, a random copolymer that includes, as polymerization units, vinylidene fluoride and hexafluoropropylene, or the like is used (for example, see U.S. Pat. No. 5,296,318). In order to improve energy density without degrading discharge load characteristics, a block copolymer that includes, as polymerization units, vinylidene fluoride and hexafluoropropylene, and the like are used (for example, see Japanese Unexamined Patent Application Publication No. 2000-123873). In order to prevent precipitous heat generation upon occurrence of internal short-circuit or the like, monomethyl maleate, or the like as a denaturation substance is added to a random copolymer that includes, as polymerization units, vinylidene fluoride and hexafluoropropylene, or the like. (for example, see Japanese Unexamined Patent Application Publication No. 2006-286496). In order to improve ion conductivity without decreasing mechanical strength, acrylic acid or the like is graft-copolymerized with a copolymer that includes, as polymerization units, vinylidene fluoride and hexafluoropropylene (for example, see Japanese Unexamined Patent Application Publication No. 2003-077539).